The disclosure relates generally to an image sensor and, more particularly, to an image sensor having enhanced absorption coefficient for improved pixel performance and a method of manufacturing the same.
Complementary metal oxide semiconductor (CMOS) image sensors are gaining in popularity over traditional charge-coupled devices (CCDs) due to certain advantages inherent in the CMOS image sensors. In particular, CMOS image sensors typically require lower voltages, consume less power, enable random access to image data, may be fabricated with compatible CMOS processes, and enable integrated single-chip cameras.
CMOS image sensors utilize light-sensitive CMOS circuitry to convert light energy into electrical energy and can be designed to be illuminated from a front surface or from a back surface. The light-sensitive CMOS circuitry typically comprises a photodiode formed in a silicon substrate. As the photodiode is exposed to light, an electrical charge is induced in the photodiode. The photodiode is typically coupled to a MOS switching transistor, which is used to sample the charge of the photodiode. Colors may be determined by placing filters over the light-sensitive CMOS circuitry.
The light received by pixels of the CMOS image sensor is often based on the three primary colors: red, green, and blue (R, G, B), and additional colors can be identified and/or created with various combinations and intensities (e.g., when red and green overlap they form yellow). FIG. 1 is a graph showing the relationships between the wavelengths of blue, green, and red light and their respective quantum efficiencies. Incident light with a long wavelength such as red light (wavelength of about 600 nm) has a decreased absorption coefficient. As pixel performance is dependent on the absorption characteristics of the photodiode, this decreased absorption coefficient results in decreased red signals and an imbalance in color ratio.
Furthermore, pixel sensitivity for receiving the incident light is lowered with the trend of size reduction of pixels of the CMOS image sensor and cross-talk is caused between different pixels with incident light, especially for incident light having a long wavelength, thereby degrading the overall performance of pixels of the CMOS image sensor.
Hence, what is needed is an improved image sensor that provides for increased absorption coefficient and therefore improved pixel performance, especially for light having a long wavelength.